1. Field of the Invention
The present invention relates to a method for notifying an FR network of traffic congestion in an ATM network.
2. Description of Related Art
ATM (Asynchronous Transfer Mode) is a new connection-oriented packet-switching technique which the international telecommunication standardization organization ITU-T has chosen to be the target solution of a broadband integrated services digital network (B-ISDN). The problems of conventional packet networks have been eliminated in the ATM network by using short packets of a standard length (53 bytes) known as cells.
FIG. 1a shows the basic structure of one cell to be transmitted in an ATM network. Each cell transmitted in the network contains a payload section of 48 bytes and a header of 5 bytes in length, but the more detailed structure of the header (the contents of the header) depends on which part of the ATM network is being used at the time. The ATM network architecture comprises a group of interfaces accurately specified in the standards, and the header structure used in an ATM cell depends on the interface (i.e. the part of the network) in question.
FIG. 1b shows the structure of the cell header at a UNI interface (User-to-Network Interface) of the ATM network, which is the interface between an ATM terminal equipment (such as a computer, a router or a telephone exchange) and an ATM node. FIG. 1c shows the structure of the header at an NNI interface (Network-to-Network Interface) of the ATM network, which is the interface between two ATM nodes, either inside the network or between two different networks.
The routing field of the cell header consists of the Virtual Path Identifier VPI and the Virtual Channel Identifier VCI. In the header structure of FIG. 1b, which is used only at a subscriber interface, 24 bits in all are reserved for the routing field (VPI/VCI). In the header structure of FIG. 1c, which is used everywhere else in the ATM network, 28 bits are reserved for the routing field (VPI/VCI). As the name suggests, the routing field serves as the basis for the routing of cells in the ATM network. The inner parts of the network primarily use the virtual path identifier VPI, which in practice often determines to which physical connection a cell is to be routed. On the other hand, the virtual channel identifier VCI is often used for routing only at the boundary of the network, for example when connecting FR connections (FR=Frame Relay) to the ATM network. However, it is to be noted that only VPI and VCI together define the route of the cell unambiguously.
The other fields in the header of an ATM cell as defined by the specifications are
GFC (Generic Flow Control): a field intended for traffic control at a subscriber interface, not yet accurately defined, PA1 PTI (Payload Type Indicator): primarily used for distinguishing between the management cells of the network and the information cells of the subscribers, but it is also possible to distinguish information cells of the subscribers on the basis of whether or not congestion has been detected on the way, PA1 CLP (Cell Loss Priority): used for prioritizing cells in relation to the discard probability (closely corresponds to the DE bit of the Frame Relay network), PA1 HEC (Header Error Control): the check sum of the header.
Of these other fields, the present invention is solely concerned with the PTI field, which can be used for transmitting congestion notifications specific for a virtual path or a virtual channel.
The Frame Relay (FR) technique is a packet-switched network technique used for the transmission of frames of varying length in place of the packet-switched network connections presently in use. The protocol (X.25) applied generally in the present packet-switched networks requires plenty of processing and the transmission equipment is expensive, as a result of which the speeds also remain low. These matters are due to the fact that the X.25 standard was developed when the transmission connections used were still rather prone to transmission errors. The starting point of the frame relay technique was a considerably lower transmission line error probability. It has therefore been possible to discard a number of unnecessary functions in the frame relay technique, which makes the delivery of frames rapid and efficient. The Frame Mode Bearer Service is described generally in ITU-T (former CCITT) Recommendation I.233 and the associated protocol in Recommendation Q.922. For a more detailed description of the FR technique, reference is made to An Overview of Frame Relay Technology, Datapro Management of Data Communications, McGraw-Hill Incorporated, April 1991, as well as the above-mentioned recommendations.
As stated above, data is transmitted in an FR network in packets of varying length, so-called frames. In FR networks, congestion is indicated by a so-called FECN or BECN (Forward or Backward Explicit Congestion Notification) bit of a frame. FIG. 2 shows a typical FR network frame format 21 where the address field preceding the information field comprises two octets (bits 1 to 8). Bits 3 to 8 of the first octet and bits 5 to 8 of the second octet form a Data Link Connection Identifier DLCI, which indicates to the node for example the virtual connection and virtual channel to which a particular frame belongs. The virtual channels are distinguished from each other by means of the data link connection identifier. The data link connection identifier, however, is unambiguous only over a single virtual channel, and it may change in the node on transition to the next virtual channel. Bits 3 and 4 of the second octet are the above-mentioned FECN and BECN bits. As the other bits in the FR frame are not relevant to the present invention, they will not be described more closely herein. An interested reader will find a more detailed description for instance in the references mentioned above.
Interconnection of ATM- and FR-type networks has been dealt with, for example, in ITU Draft Recommendation I.555 and B-ICI Specification, Version 1.0, June 1993. In order to deal with congestion at an interface between networks, it has been suggested that the PTI bit (which is the middle bit of the three-bit PTI field) from the ATM network be picked from the last cell of a segmented frame and encoded into the FECN bit of the FR network, whereby information of congestion is transferred from the ATM network to the FR network (B-ICI 1.0, p. 116).
The drawback of this procedure is that the FECN and BECN bits are part of an FR frame of normal traffic, wherefore the delivery of a congestion notification may be delayed or even completely fail. The cell carrying a congestion notification in the ATM network may also be caught in congestion, and thus the propagation of the congestion notification may be delayed. FR traffic has been managed using a procedure according to which, having received a FECN bit as an indication of congestion, a terminal equipment transmits information to the other end, using a BECN bit. In order that this bit might be transmitted, there must however be subscriber traffic in this direction. This drawback will be illustrated more closely below in connection with FIG. 4.